Audio equalization means modifying the frequency balance of sound by attenuating or emphasizing the magnitude at certain frequencies. The motivation of this task is, e.g. enhancement of music listening experience, correction of the magnitude response of a sound output device (such as headphones or loudspeaker reproduction systems), or equalization of a room response. In music player and audio editing software, it is common to enable the listener to modify the frequency balance of a sound through a graphical user interface, denoted as a graphic equalizer.
A graphic audio equalizer (graphic audio EQ) enables visual and usually interactive way of frequency balance modification of audio in real time, and by means of digital signal processing. The available frequency region (e.g., from zero to Nyquist frequency) is divided to a certain number of bands whose magnitudes can interactively be modified. FIG. 1 depicts a target EQ magnitude response curve of an 8-band equalizer. The criteria according to which the EQ curve should be followed depend on implementation requirements. For example, in some cases it may be desirable to have as steep magnitude transitions as possible between adjacent bands. Usually, however, the target is to match the given magnitudes only at center frequencies of each band and have smooth magnitude changes between bands.
Typically, the EQ bands are distributed logarithmically (e.g., in octave or third octave bands) or in some other non-uniform manner in the frequency domain. Logarithmic distribution of bandwidths yields to narrow bands at low frequencies and wider bands at high frequencies. This type of EQ band distribution is well justified by the characteristics of human hearing, where the frequency resolution roughly follows the logarithmic scale.
A typical equalizer implementation includes cascaded peak and shelving filters, where the output magnitude response is calculated as a product of the magnitude responses of the cascaded filters. Another option is to connect the filters in parallel, in which case the resulting EQ magnitude response is the sum of the responses of the filters in the parallel connection. In the latter case, a problem may rise from different phase responses of the filters. In both cases, the gain adjustments of each band can be implemented by varying the parameters of only one filter.
There are different ways to implement a cascade of peak- and shelving filters. For example, in publication “Tunable Digital Frequency Response Equalization Filters” by P. A. Regalia and S. K. Mitra, IEEE Transactions on Acoustics, Speech, and Signal Processing, Vol. ASSP35, No. 1, January 1998, a solution is proposed where the output from an allpass filter is mixed with the direct signal. Concerning the above-mentioned parallel filtering it is disclosed in U.S. Pat. No. 5,892,833 that it is possible to achieve a low group delay as well as a good approximation to the target EQ magnitude response by adding together the outputs from a number of infinite impulse response (IIR) filters.
However, the above-mentioned solutions for equalization are not very suitable when the bandwidths of the subbands of the target EQ magnitude are very different, as in this case, the computational complexity of the filters increases, especially when FIR filters are applied for equalization. The usage of IIR filters, as proposed U.S. Pat. No. 5,892,833, would decrease said computational complexity, but, on the other hand, would introduce chirp-like audible artifacts caused by non-linear phase responses, and, furthermore, said IIR filters are extremely sensitive to noise and round-off errors.